The invention relates to a multilayer, biaxially oriented polypropylene film comprising a base layer and at least one heat-sealable top layer and at least one interlayer in accordance with a BZD layer structure, which film comprises wax in its interlayer.
The improvement in the barrier properties of films, in particular of films for the packaging sector, has recently increased in importance. For cost and environmental reasons, the packaging industry desires ever thinner films which have the same or improved barrier properties, in particular with respect to the passage of water vapor.
The barrier action of BOPP films to water vapor (VWBA) and oxygen (OBA) decreases with the film thickness. In the usual thickness range of BOPP films (from 4 to 100 xcexcm) there is an approximately hyperbolic relationship between the water vapor barrier action (WVBA) and the thickness (d) (WVBAxc3x97d=const). The constant depends essentially on the raw material composition and the stretching conditions. For BOPP packaging films in accordance with the prior art the constant has a value of approximately: const.=28 gxc3x97mm/m2xc3x97d. The water vapor permeability here has been measured in accordance with DIN 53 122.
U.S. Pat. No. 4,921,749 (EP-A-0 247 898) describes a heat-sealable BOPP film having improved mechanical and optical properties. The heat-sealability of the film and the water vapor and oxygen permeability are likewise improved. All the improvements result from the addition of a low-molecular-weight resin to the base layer. The resin content here is between 3 and 30% by weight The resin has a molecular weight of significantly less than 5000, preferably less than 1000, and is, for example, 600. The softening point of the resin is from 120 to 140xc2x0 C.
U.S. Pat. No. 5,155,160 describes the improvement in the barrier properties by the addition of wax to unoriented polypropylene films. The waxes described are paraffin waxes and polyethylene waxes having a molecular weight of from 300 to 800. The barrier action is said to be less than 0.2 g/100 square inches/24 hours.
There is a continuous demand for a further improvement in the water vapor barrier action of biaxially oriented packaging films made from polypropylene. The methods disclosed hitherto fail to reduce the water vapor barrier action to the desired extent or impair other essential film properties in an unacceptable manner.
The object of the present invention was therefore to provide a biaxially oriented polypropylene film which is distinguished by a good barrier action, in particular to water vapor, and has good mechanical properties. It must be possible to produce the film with reliable running and operation at production speeds of up to 400 m/min. Other physical film properties required in view of their use as packaging film must not be adversely affected. The film should have high gloss, no optical defects in the form of fisheyes or bubbles, good scratch resistance, fault-free running on high-speed packaging machines at low film thickness, and, for transparent embodiments, low film haze. In addition, the heat-sealing properties must not be adversely affected.
This object is achieved in accordance with the invention by a multilayer polypropylene film of the generic type mentioned at the outset, wherein the interlayer comprises a wax having a mean molecular weight (number average) of from 200 to 1200.
In a preferred embodiment, the film consists of a base layer B, interlayers Z applied to both sides thereof, and top layers D applied to the interlayers, i.e., a five-layer symmetrical structure DZBZD. In a further preferred embodiment, the film consists of a base layer B, an interlayer Z applied to one side thereof, and top layers D applied to the base layer and the interlayer In accordance with DBZD. If desired, these basic structures comprising three, four or five layers may contain further interlayers.
The base layer of the film generally comprises at least 70 to 100% by weight, preferably from 75 to 98% by weight, in particular from 80 to 95% by weight, in each case based on the base layer, of a propylene polymer described below.
This propylene polymer comprises at least 90% by weight, preferably from 94 to 100% by weight, in particular from 98 to 100% by weight, of propylene. The corresponding comonomer content of at most 10% by weight or from 0 to 6% by weight or from 0 to 2% by weight respectively generally consists, if present, of ethylene. The data in percent by weight in each case relate to the propylene homopolymer.
Of the above-mentioned propylene polymers, isotactic propylene homopolymers are preferred for the base layer. In general, the propylene homopolymer has a melting point of from 140 to 170xc2x0 C., preferably from 150 to 165xc2x0 C., and a melt flow index (measurement DIN 53 735 at a load of 21.6 N and 230xc2x0 C.) of from 1.5 to 20g/10 min, preferably from 2 to 15g/10 min. The n-heptane-soluble content of the polymer is generally from I to 6% by weight, based on the polymer.
In a preferred embodiment of the invention, the propylene homopolymer employed is highly isotactic. For highly isotactic propylene homopolymers of this type, the chain isotacticity index of the n-heptane-insoluble content of the polypropylene, determined by 13C-NMR spectroscopy, is at least 95%, preferably from 96 to 99%.
In a further preferred embodiment of the film according to the invention, the propylene homopolymer of the base layer has been peroxidically degraded.